// ColoredPoints.js
// Vertex shader program
var VSHADER_SOURCE =
'attribute vec4 a_Position;\n' +
'void main() {\n' +
' gl_Position = a_Position;\n' +
' gl_PointSize = 10.0;\n' +
'}\n';

// Fragment shader program
var FSHADER_SOURCE =
'precision mediump float;\n' +
'uniform vec4 u_FragColor;\n' + // uniform variable <- (1)
'void main() {\n' +
' gl_FragColor = u_FragColor;\n' + //<- (2)
'}\n';

function main() {
    // Retrieve <canvas> element
    var canvas = document.getElementById('webgl');

    // Get the rendering context for WebGL
    var gl = getWebGLContext(canvas);
    if (!gl) {
        console.log('Failed to get the rendering context for WebGL');
        return;
    }
	// Initialize shaders
	if (!initShaders(gl, VSHADER_SOURCE, FSHADER_SOURCE)) {
        console.log('Failed to initialize shaders.');
        return;
	}

	// Get the storage location of a_Position variable
	var a_Position = gl.getAttribLocation(gl.program, 'a_Position');
    if (a_Position < 0) {
        console.log('Failed to get the storage location of a_Position');
        return;
    }

	// Get the storage location of u_FragColor variable
	var u_FragColor = gl.getUniformLocation(gl.program, 'u_FragColor');
    if (u_FragColor < 0) {
        console.log('Failed to get the storage location of u_FragColor');
        return;
    }

	// Register function (event handler) to be called on a mouse press
	canvas.onmousedown = function(ev){ click(ev, gl, canvas, a_Position,u_FragColor) };

    // Pass vertex position to attribute variable
    gl.vertexAttrib3f(a_Position, 0.0, 0.5, 0.0);

 	// Set the color for clearing <canvas>
    gl.clearColor(0.0, 0.0, 0.0, 1.0);

    // Clear <canvas>
	gl.clear(gl.COLOR_BUFFER_BIT);
}

var g_points = []; // The array for a mouse press
var g_colors = []; // The array to store the color of a point
function click(ev, gl, canvas, a_Position, u_FragColor) {
	var x = ev.clientX; // x coordinate of a mouse pointer
	var y = ev.clientY; // y coordinate of a mouse pointer
	var rect = ev.target.getBoundingClientRect();

	x = ((x - rect.left) - canvas.width/2)/(canvas.width/2);
	y = (canvas.height/2 - (y - rect.top))/(canvas.height/2);

	// Store the coordinates to g_points array
	g_points.push([x, y]);
	// Store the color to g_colors array
	if(x >= 0.0 && y >= 0.0) { // First quadrant
		g_colors.push([1.0, 0.0, 0.0, 1.0]); // Red
	} else if(x < 0.0 && y < 0.0) { // Third quadrant
		g_colors.push([0.0, 1.0, 0.0, 1.0]); // Green
	} else { // Others
		g_colors.push([1.0, 1.0, 1.0, 1.0]); // White
	}

	// Clear <canvas>
	gl.clear(gl.COLOR_BUFFER_BIT);

	var len = g_points.length;
	for(var i = 0; i < len; i++) {
		var xy = g_points[i];
		var rgba = g_colors[i];

		// Pass the position of a point to a_Position variable
		gl.vertexAttrib3f(a_Position, xy[0], xy[1], 0.0);
		// Pass the color of a point to u_FragColor variable
		gl.uniform4f(u_FragColor, rgba[0],rgba[1],rgba[2],rgba[3]); //<-(3)
		// Draw a point
		gl.drawArrays(gl.POINTS, 0, 1);
	}
}